Dual estradiol action in diencephalon and the regulation of sociosexual behavior in female golden hamsters

Previously, we found that single implants of estradiol (E₂) placed in the ventromedial hypothalamus (VMH) but not the anterior hypothalamus (AH) facilitated precopulatory, i.e. vaginal scent marking, and copulatory, i.e. lordosis, behavior following progesterone administration. However, the duration of lordosis was markedly attenuated in comparison to the duration shown by intact cycling females. This study was designed to examine whether dual implantation of E₂ in the diencephalon would facilitate patterns of precopulatory and copulatory behaviors similar to those shown by intact cycling females. One E₂ implant was placed in either the medial preoptic area (MPO) or AH and a second E₂ implant was placed in the VMH. Control females were tested following E₂ application at only the MPO or AH region in conjunction with a cholesterol implant in the VMH. An additional control group was tested with females implanted with cholesterol at both MPO-AH regions and the VMH. During a 2-day postimplantation test period, vaginal marking scores were elevated for both single and dual E₂ implanted females and agonistic response patterns toward males declined significantly. In addition, a significant inverse relationship was found between the number of vaginal marks and bites exhibited by females with single E₂ implants in the MPO, whereas these two response patterns were positively correlated in females with E₂ stimulation occurring only in the AH region. No significant relationship was found between vaginal marking and biting attack for females receiving dual E₂ stimulation. Systemic progesterone administration on the third postimplantation day facilitated sexual receptivity in the majority of females with dual E₂ implants (>90%). These receptive females displayed lordotic responsiveness that closely matches the full display of sexual receptivity shown by intact cycling females. In contrast, only one female with a single E₂ implant in the AH region showed sexual responsiveness. The results demonstrate that: (1) precopulatory vaginal marking and biting attack are mediated by E₂ action in the MPO and AH but in a different manner; (2) additional action of E₂ in the VMH diminishes the distinctive precopulatory behavioral effects of E₂ in the MPO and AH suggesting an influential role of the VMH in regulating sociosexual activities; and (3) E₂ action in either the MPO or AH region in conjunction with E₂ action in the VMH may be necessary in order to facilitate the species-typical-display of lordotic responsiveness.